AME 36:1-8 (2004)  -  doi:10.3354/ame036001

Evidence of Trichodesmium viral lysis and potential significance for biogeochemical cycling in the oligotrophic ocean

Ian Hewson1,*, Sarah R. Govil2, Douglas G. Capone1, Edward J. Carpenter2, Jed A. Fuhrman1

1Department of Biological Sciences, University of Southern California, 3616 Trousdale Parkway, AHF 107, Los Angeles, California 90089-0371, USA
2Romberg Tiburon Center, San Francisco State University, 3152 Paradise Drive, Tiburon, California 94920, USA

ABSTRACT: The planktonic cyanobacterium Trichodesmium spp. is a globally important diazotroph, fixing at least 80 Tg N yr-1 in tropical waters. Despite its biogeochemical importance, the mechanisms of Trichodesmium mortality, and the means by which its fixed N enters upper levels of the food web, are poorly understood. Potential virus-like particle (VLP) production by Trichodesmium spp. was observed in both culture (IMS101) and field samples from the tropical North Pacific Ocean, in oceanic waters around the Hawaiian Islands. VLP observed by TEM in IMS101 lysate were approximately 56 nm wide and untailed, and VLP with similar morphology were observed in tissue treated with mitomycin C. A most-probable number cultivation technique (utilizing bacterized cultures of Trichodesmium) detected moderate abundances of Trichodesmium-infecting cyanophage (605 to 9750 ml-1 infecting only 1 cultured strain) in 0.2 μm-filtered seawater samples from surface, subsurface and deep chlorophyll maximum samples. Estimation of mortality from virus production was not possible due to rapid VLP release in the first 6 h of dilution incubations. Rather, an indirect approach using burst size (determined from mitomycin C treatment of washed trichomes resuspended in virus-free seawater), decay rate of VLP (from latter part of virus production incubations) and average cyanophage titer was used to estimate mortality. These conservative calculations suggested that 0.3 to 6.5% d-1 (mean = 1.65 ± 0.99% d-1) of trichomes could potentially be lysed by viruses, representing the release of approximately 3 to 64% fixed N d-1. These estimates are based upon a steady-state maintenance of observed VLP abundance, which in nature could be from lysogeny, pseudolysogeny, carrier state, or successive lytic infection. Viral lysis therefore may represent a significant novel mechanism of N release from Trichodesmium spp., even in non-bloom conditions.

KEY WORDS: Trichodesmium · Cyanophage · Nitrogen · Diazotroph · Lysogeny

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